1. Senior/Master Air Cadet
PILOT NAVIGATION Part 3
Senior/Master Air Cadet

2. THE WEATHER

3. Introduction
Previously you have learnt how the weather affects the hill walker, and the aviator in the circuit
We will now see how the weather affects aircraft en route between airfields

5. The major variable is water as we shall see
The Air
In order to understand the reactions involved when the air is in motion we must consider its constituents
The major variable is water as we shall see

6. Pure air consists of : OTHER 1 % (POLLUTION, OZONE, CO 2) OXYGEN 20%
NITROGEN 79%

7. Temperature & Pressure
From your physics you will remember Boyles’s gas laws. If you don’t you should know that up high in the atmosphere (such as on top of a mountain) it is very cold and climbers need to carry oxygen
This is because the air pressure is very low at high altitudes

8. Temperature & Pressure
The air pressure at sea level is caused by the weight of the air above us. The higher you go, the less the weight becomes
If a gas is compressed it becomes hotter, & so the less compressed it is the colder it is

9. Water Vapour
A certain volume of air, under fixed conditions of pressure and temperature, can only hold a certain amount of water vapour, which is an invisible gas
If this air becomes cooler it will not be able to hold as much moisture and will eventually become saturated with water vapour

10. When the air cools to this point it is known as the dew point
Water Vapour
When the air cools to this point it is known as the dew point
It is at this point that dew , mist or fog will form. If the temperature falls below freezing then frost or freezing fog will occur

11. Cloud
When unsaturated air rises the temperature drops & the dew point is reached, and cloud is formed

12. D P
AIR RISING
DUE TO
TRIGGER
ACTION
TEMPERATURE
DROPPING

13. There are four trigger actions that force air to rise:
Turbulence - air currents coming together
Convection - heating of the air by the sun
??????

14. Trigger Actions
Orographic uplift (mountain effect) air hitting mountain ranges

15. Frontal uplift - when 2 fronts meet
Trigger Actions
Frontal uplift - when 2 fronts meet
Warm Front
Warm air
Cool Air

16. The height of the cloud will depend on the amount of moisture in the air, and the strength of the uplift
In extreme cases Cumulo Nimbus thunder clouds form, and these are a severe hazard to aircraft

17. Cumulo Nimbus

18. Cumulus
At high levels the clouds (cirrus) consists of ice crystals, but most other clouds are formed by visible droplets of water

19. Precipitation - all types are present - the most dangerous being hail
Thunderstorms
A large thunderstorm presents a variety of hazards, both in the cloud and surrounding areas, to aircraft and are best avoided
Precipitation - all types are present - the most dangerous being hail
Icing - as previously covered

20. the main effect is psychological. Lighting often strikes aircraft
Lighting & thunder
the main effect is psychological. Lighting often strikes aircraft
but the main danger is to electrical systems- radar, radios etc, and temporary blinding of the crew !

22. Turbulence
the air in a thunder cloud can be in vertical motion up or down in excess of 50 knots and can change from up to down and back again in seconds. This can destroy aircraft -

24. Landing hazards
all of the above hazards exist under the base of the thunderstorm - the most significant being the risk of a severe down draught just as the aircraft is landing

25. Most modern aircraft carry weather radar’s for detecting thunderstorms & turbulent air

26. The normal range of pressure is
Isobars
When watching the weather forecasts on TV you will have noticed that the air pressure changes from place to place
The normal range of pressure is
930 millibars
to 1050 millibars

27. The pressure is shown on the chart by
Linking areas of
equal pressure by a line called an Isobar
shown thus
1016

28. 1016
1000
992 L
The isobars surround areas of high or low pressure, and show us how the wind is moving - the wind velocity (WV)

29. For example a WV of 200/25 is a wind coming from 200° at 25 knots
The WV is always expressed as where it is coming from in degrees and its strength in knots
For example a WV of 200/25 is a wind coming from 200° at 25 knots

30. In the northern hemisphere the wind circulates clockwise around anti cyclones (high pressure areas) & anti clockwise around cyclones ( low pressure areas)
1016
1000
992 L
WIND

31. This is reversed in the southern hemisphere
To remember this stand with your back to the wind, and the area of low pressure is to your left
This is reversed in the southern hemisphere

32. Isobars – facts and figures
Isobar patterns represent the wind at 2000’ above the surface

33. Isobars – facts and figures
The direction of the lines give the direction the wind
The closer the lines the stronger the wind

34. Isobars – facts and figures
At the surface the wind is about 25% weaker than at 2000’ as a result of surface friction.
It is also backed by about 25°
For example: a wind at 2000’ of 270/20 will be 245/15 at the surface

35. TAFs & METARs
The weather forecaster uses many charts & symbols to convey the details of the weather over the whole country
For an aviator, who receives info from radio, it must be coded & is standardized world wide (except Canada !)

36. Terminal Aerodrome Forecast
There are 2 formats:
T A F
Terminal Aerodrome Forecast
M E T A R
Meteorological Actual Report
one records a forecast and the other reports actual conditions

37. TAF DECODE

38. TAF Normally published for a 24 hour period but may be shorter.
Airfield Identifier.
Time Published
Time Valid For

39. TAF
Wind
Visibility
Significant Weather

40. TAF
Cloud
Significant Changes

41. TAFs do not include temperatures or pressures

42. The Code
TAF
CRANWELL 1306/ / HZ SCT030 BECMG CAVOK=

43. This translates as
Cranwell/ for 0600 hrs on 13th hrs on 14th/ surface wind 260° true at 5 knots/ visibility 4000 meters in haze/ scattered cloud base at 3000’/ becoming “cloud and visibility okay” that is the visibility will be at least 10km and no cloud below 5000’
CRANWELL / HZ SCT030 BECMG CAVOK=

44. Gives the conditions at an airfield and is recorded hourly
METAR
Gives the conditions at an airfield and is recorded hourly
It is given to aircraft inbound
It is normally prefixed by a time - which is the time the conditions were observed
It can also be designated by a letter code,
ATIS(Automatic Terminal Information Service)
‘A’ would be the first METAR for the airfield
when it opens and would move through the
alphabet hourly.

45. If the weather changes rapidly a SPECI – (special) report is issued
METAR
If the weather changes rapidly a SPECI – (special) report is issued
METARs & SPECIs Do not forecast conditions, but do include pressure & temperature

47. METAR
Identification
Wind
Visibility

48. METAR
RVR
Only given if visibility is poor
Present Weather
Cloud

49. METAR
CAVOK
Temperature and Dew Point
QNH
Recent Weather

50. METAR
Wind Shear (If present)
Colour States
Runway State Group

51. METAR
Trend

52. The Code
METAR
CRANWELL / FEW / =

53. This translates as
CRANWELL/ AT 0900 Z HRS/ SURFACE WIND 250° TRUE AT 7 KNOTS/VISIBILITY 8000 METERS/LOWEST CLOUD - few (EIGHTS) AT 3500’/TEMPERATURE +17°C, DEW POINT + 13°C/PRESSURE 1028
CRANWELL / FEW / =

54. In the USA temperature is give in °F
= Means End Of Message
Pressure is the QNH at the airfield

55. Check of understanding
What is a TAF?
Terminal Aerodrome Forecast

56. Meteorological Actual Report
What is a METAR?
Meteorological Actual Report

57. If the weather changes rapidly a SPECI – (special) report is issued
What is a SPECI?